The simple answer is: There’s no simple answer. Extensive scientific research on the effects of 8- and 12-hour shifts has produced no clear winner. Both 12s and 8s work wonderfully for some facilities while causing problems for others.

As further evidence that one schedule is not clearly better than the other, look at the distribution of shift schedules in North America. In a CIRCADIAN survey of 400 shift work operations, 37 percent were using 12-hour shifts while 34 percent were using 8-hour shifts (see graph below).

We do know this: The “perfect shift schedule” for each facility depends on a large number of factors, including business needs, the nature of the job tasks performed, and workforce demographics and preferences.

The Research: 8s vs. 12s

A research review of “8s vs. 12s,” which examined over 60 research studies, rated 8s and 12s in terms of performance, safety, health, worker satisfaction, absenteeism and overtime (Smith L, et al, 1998). Here’s what the review found:

Performance. The evidence comparing the effects of 8s and 12s on job performance is unclear. Several studies of nurses concluded that there are generally no differences in fatigue or critical thinking performance in those working either shift schedule. However, three studies found that nurses’ performance tends to fare worse when they’re on 12s than on 8s. One of these three studies reported that the reduced performance appeared to be acute and related to the first 12-hour shift after a block of days off.

Two studies found that performance stayed the same within the mining, chemical, and petroleum industries.

Several studies mentioned that 12s are popular among shiftworkers because they compress the work week and provide more days-off than 8’s. Several studies attributed the popularity of 12s as improving staff morale and reducing absenteeism.

Additionally, the popularity of 12’s also been hypothesized to increase worker motivation and stimulate greater effort to reduce any possible detrimental effects of increased fatigue on 12-hour schedules.

Safety. The vast majority of studies that compared accident and error rates before and after a schedule change found no difference between 8s and 12s. Studies on this topic have included nuclear power plants, petrochemical companies, fertilizer producers, utilities, and processing plants.

In one study, a decrease from three to two shift handovers per day following a switch from 8s to 12s was thought to have reduced operator error. On the other hand, two studies cited accumulated sleep debt and recommend against 12-hour shifts, especially schedules entailing more than three to four 12-hour shifts in a row.

Health. Most studies have found few differences between 8s and 12s when it comes to physical health, social well-being, and sleep. Studies that examined worker satisfaction with family life favored 8-hour shifts for nurses and 12-hour shifts for police and chemical workers.

A shift schedule’s effect on health and social issues will vary considerably based on individual differences, which is why it’s important to consider the characteristics of each worker population during schedule selection.

Facilities in various industries reported positive results when employees participated in negotiating changes such as shift start and end times, provision of an extra break during 12-hour shifts, and improved meal facilities at night. At one plant in which the schedule was imposed with no employee input, worker reactions were so negative that the old schedule had to be reinstated.

Absenteeism. Six studies found no difference in absenteeism rates between 8s and 12s. One plant saw an increased applicant rate and decreased turnover after switching to 12s. However, two studies found that older workers had less favorable attitudes toward 12-hour shifts, leading to higher absence rates among older workers who participated in that study.

Overtime. The amount of overtime does not generally differ between 8s and 12s. However, companies using 12-hour shifts may want to adjust overtime policies so that workers are not held over for dangerously long periods. Several studies suggest that safety incidents go up when the average annual overtime worked by employees goes up, an effect thought to be produced by fatigue from working long hours.

In addition, a 12-hour shift schedule often allows employees greater opportunities for second jobs; one study found 25 percent of shiftworkers on 12-hour schedules were moonlighting, which may take energy away from their primary jobs.

Conclusions

In summary, to quote the authors of the research review, “The research findings are largely equivocal. The bulk of the evidence suggests few differences between eight and 12 hour shifts in the way they affect people.”

When reading these study results it’s important to remember that one study’s conclusion about (for example) a “2-3-2” shift pattern may not apply to all 12-hour shift patterns. For both 8s and 12s, there are hundreds of different shift patterns for 24/7, 24/6 and 24/5 facilities, and each study typically only examines one pattern.

That’s one reason why it’s difficult to draw conclusions about whether 8s or 12s are best. You can’t compare apples to apples if two studies of 12-hour shifts each look at a different shift pattern — or a different industry — or a different number of employees — or any of dozens of variable factors.

So which schedule is right for your operation? A thorough analysis of your company’s own needs and your workers’ preferences will let you determine the optimum schedule for your facility. To learn more, please visit our Shift Scheduling Optimization Process page or call us at 1-800-284-5001 or 781-439-6300.